The ever increasing miniaturization of electronic components simultaneously leads to an increase in the integration density. In printed circuit boards the trend towards miniaturization is reflected in the following construction parameters: reduction of pad diameters and conductor width/conductor gap as well as improved registration and increase in the number of layers (cf. Der europäische Technologie- und Trendbericht 2001/2002 Ober Leiterplatten mit hohen Integrationsdichten1). 1The European Technology and Tred Report 2001/2002 Concerning Printed Circuit Boards with High Integration densities
Printed circuit boards having these properties are generally referred to as printed circuit boards with high integration density (so-called High Density Interconnection or HDI).
An important aspect in such HDI circuits in printed circuit board production is the filling of through-holes (so-called via-holes). The filling of the through-holes puts extremely high requirements on process control. The most different types of drill holes have to be taken into account, the various requirements on the filler materials have to be met and the subsequent working steps in a printed circuit board have to be taken into account.
The main focus of the present invention is the filling of through-holes in printed circuit boards which go through the entire board (Plated Through Hole, PTH) and of interior vias (buried vias).
In principle, the process is suitable for filling through-holes in the most different workpieces, in particular board-shaped workpieces and board-shaped electric circuit carriers containing through-holes.
The closing of the through-holes is necessary inter alia so as to prevent the deposition of solder on the components, to achieve a high integration density and to improve the electrical properties. In multi-layer printed circuit boards inclusions (of air, solvent, etc.) in the holes might occur during the laminating of the next build-up layer, which inclusions later on in the case of thermal stresses lead to bulges and, consequently, cracks in the next layer.
Thus, the main requirements to be fulfilled by the filler materials for through-holes are:                the absence of solvents        good adhesive properties to the sleeve and solder resist        resistance to process chemicals in subsequent steps (e.g. galvanic metallization with nickel, gold or tin).        resistance in hot air leveling processes.        
In the state of the art various processes for filling through-holes are described.
In the simplest case the holes are filled using a specially adjusted solder resist. They have the advantage to offer that in the case of high integration density no impairment is caused in the resolution by the via filler which necessarily protrudes like a rivet head. However, what is disadvantageous is the danger of solvent inclusions which can abruptly evaporate in subsequent process steps, such as tinning, and, thus, tear open the cover.
However, this process is not suitable for closing through-holes in inner layers. Here, the inner layers have to be completely closed in order to avoid inclusions. For this process plugging is widely used because by means of this process it is possible by means of copper-plating the filled through-holes to create an inner layer which can be structured without any limitations.
As filler material various dielectrics such as resin-coated copper foils (RCC) or photo-dielectric liquid or dry films are used.
EP 0 645 950 B1 describes a process for producing multi-layered circuit substrates. As filler material for through-holes thermosetting resins are used chosen from the group consisting of phenolic resin and epoxy resin. Furthermore, as a conductive substance at least one metal powder chosen from the group consisting of silver, nickel, copper and an alloy thereof is added to the resin.
As a rule, the plugging is done after the printed circuit board has been drilled and the drill holes finally metallized, however, before the structuring. After the vias have been filled and the plugging paste has been cured, the latter is mechanically leveled since due to the filling process it exhibits a slight rivet head. Often, a metallization of the paste with copper is subsequently carried out so that a continuous copper layer is created as final layer. To put it simply, the following steps are required:                drilling        metallizing the sleeve        plugging        brushing, grinding        metallizing the plugging paste        applying the next build-up layer.        
EP 1 194 023 A1 describes the manufacturing of HDI printed circuit boards by filling through-holes with conductive pastes, wherein the curing of the paste can occur at the same time as the molding of the basic material so that an electric contact of interior layers results.
However, the processes involve much outlay and cause a great deal of the process costs in the manufacture of HDI printed circuit boards. Furthermore, for each layout on the printed circuit board a different printing mask has to be used. Therefore, the process cannot be universally used.
In the case of highly different drill hole diameters on the printed circuit board the printing process is difficult.